Molecular separation is an important process utilized in various industries, particularly in the production of fuels, chemicals, petrochemicals and specialty products. The separation of molecular compounds can be accomplished by a variety of methods that, assisted by heat, solids, or other means, generally exploits the differences in physical and/or chemical properties of the components to be separated. For example, gas separation can be achieved by partial liquefaction or by utilizing a solid adsorbent material that preferentially retains or adsorbs a more readily adsorbed component relative to a less readily adsorbed component of the gas mixture, or by several other gas separation techniques known in the industry. Conversely, some molecular compounds can be separated by exploiting differences in the density, boiling points, melting points of the compounds, or by the selective chemisorption, physisorption, or solubility of certain compounds with selected solvent materials.
However, in recent years, zeolitic materials have proven to be exceptionally valuable materials in the field of molecular separations processes. Due to their high uniformity in pore size and crystalline structure, as well as their ability to withstand a wide range of operational conditions, zeolites and in particular adsorbent matrixes containing zeolites, are often materials of choice for use in selective molecular separations processes. Molecular separations processes such as Pressure Swing Adsorption (PSA), Temperature Swing Adsorption (TSA), Pressure/Temperature Swing Adsorption (PTSA), Rapid Cycle Pressure Swing Adsorption (RCPSA), Rapid Cycle Temperature Swing Adsorption (RCTSA), as well as Selective Membrane Separation processes are all well known in the art. In preferred embodiments of these processes, many of these processes utilize zeolites as part of the adsorbent matrixes due to their pore diameters in the microporous ranges (i.e., typically less than about 2 nm) which enables the zeolites to separate molecules and compounds at the molecular level. These processes utilizing zeolites as selective molecular adsorbents or “sieves” have rapidly gained prominence in modern industrial separations use also due to their relatively low cost, high molecular selectivity, high storage capacity, high surface areas, reasonable stabilities, and ease of regeneration among other beneficial properties.
Zeolites, possessing micropores of diameter <2 nm within their framework structures, have been widely used in separation processes and shape-selective catalysis, as well as in emerging applications such as mixed-matrix separation membranes and low-k dielectric materials among others. By incorporating covalently bound (as opposed to conventionally physisorbed) organic groups within the micropores, these highly ordered materials could be converted to organic-inorganic hybrids with potential for a diverse range of new applications, made possible by variations in structure and functionality of the incorporated organic moieties.
Significant progress has been made in the organic functionalization of ordered mesoporous materials (having larger pore diameters in the 2-50 nm range), ever since the first of such materials were reported in the 1990s. Functionalization of mesoporous materials is often achieved either by one of two routes: (i) direct synthesis via the sol-gel process, involving the co-condensation of organotrialkoxysilanes R—Si(OR′)3 or organochlorosilanes R—SiCl3 with the tetraalkoxysilanes (Si—(OR)4) that are the primary silica source for mesoporous material formation or (ii) post-synthesis modification via grafting the mesoporous material with silane coupling agents such as NH—(SiR)2, Cl—SiR3, or RO—SiR′3. However, both of these approaches present difficulties in their application to zeolite materials. Direct co-condensation has been shown to lead to organic-functionalized zeolites in only rare cases, whereas the grafting of organic groups to the internal surfaces of zeolites using silane coupling agents is impeded by the small pore size of the zeolite (in many cases smaller than the molecular size of the coupling agents). When the latter technique is applied to functionalize the external surface, it also may lead to a decrease in accessible micropore volume due to the partial blockage of micropore entrances by the coupling agents and their reaction products.
Alternatively, the direct reaction of organic molecules such as alcohols with zeolite surfaces is attractive as a functionalization technique. It was reported by Iler that the esterification reaction of alcohols on silica particles can convert their hydrophilic external surfaces into hydrophobic surfaces (see Iler, R. K. The Chemistry of Silica; John Wiley & Sons: Toronto, Canada, 1979). Ballard et al. further investigated the reaction conditions for different alcohols, showing that the esterification of low-boiling alcohols on silica could be performed under autoclave conditions, whereas that of high-boiling alcohols could be conducted at elevated temperatures under ambient pressure and reflux conditions (see Ballard, C. C.; Broge, E. C.; Iler, R. K.; St. John, D. S.; McWhorter, J. R. J. Phys. Chem. 1965, 50, 20). More recently, Ishikawa et al. employed the esterification reaction of 1-butanol to probe the silanol group (Si—OH) density on the surface of mesoporous materials (see, Ishikawa, T.; Matsuda, M.; Tasukawa, A.; Kandori, K.; Inagaki, S.; Fukushima, T.; Kondo, S. J. Chem. Soc., Faraday Trans. 1996, 92, 1985). Others have applied the esterification reaction to tune the interlayer spacing of layered silicates. In addition, alcohol-functionalized mesoporous materials could further react with Grignard reagents (R—MgX, X ) Cl or Br), resulting in the formation of Si—C bonds on the internal surfaces of mesoporous materials. The hydrophobicity of the mesoporous materials treated with aliphatic alcohols was characterized by water adsorption measurements, showing that the water adsorption capacity decreased significantly after alcohol treatment. Mesoporous materials treated with aliphatic alcohols also demonstrated improved dispersion in organic solvents, possibly resulting from an increased hydrophobicity. In addition, it was found that the hydrophobicity increased with the alkyl group length and the alkyl group number density (the extent of esterification) (see, Ossenkamp, G. C.; Kemmitt, T.; Johnston, J. H. Chem. Mater. 2001, 13, 3975). However, the slow hydrolysis of alkoxy groups on esterified materials was also observed during water adsorption measurements, resulting in hysteresis during desorption (see, Kimura, T.; Kuroda, K.; Sugahara, Y.; Kuroda, K. J. Porous Mater. 1998, 5, 127).
On the other hand, the post-synthesis organic functionalization of zeolites—specifically of their internal pore surfaces—has been much less explored. For example, previous authors have reported the covalent attachment of methanol to the internal surfaces of pure-silica MFI zeolite, ascertained by a combination of techniques such as thermogravimetric analysis (TGA), IR, and NMR spectroscopy (see, Bosacek, V.; Klik, R.; Genoni, F.; Spano, G.; Rivetti, F.; Figueras, F. Magn. Reson. Chem. 1999, 37, S135; see also, Genoni, F.; Casati, G. P.; Buzzoni, R.; Palmery, S.; Spano, G.; Dalloro, L.; Petrini, G. Collect. Czech. Chem. Commun. 1997, 62, 1544; see also, Pelmenschikov, A. G.; Morosi, G.; Gamba, A.; Zecchina, A.; Bordiga, S.; Paukshtis, E. A. J. Phys. Chem. 1993, 97, 11979; see also, Bosacek, V. J. Phys. Chem. 1993, 97, 10732). It was suggested that methanol reacted with the silanol defect sites located in the zeolite structure forming ≡Si—O—CH3 linkages. The methyl group is small in size and can be expected to have only a limited effect in altering the properties of the material.
There is a need in the art for improved zeolitic materials for use in swing adsorption and/or membrane processes for the selective separation of molecular compounds. In particular, there is a need in the art for improved zeolitic materials which can be selectively modified and tailored for use in swing adsorption and/or membrane processes for the selective separation of specific molecular compounds.